Many years ago surveyors used what is now classified as relatively crude instruments for performing land surveying. As part of this effort the surveyor used a theodolite or transit for determining the horizontal or azimuth angles as well as the vertical or elevation angles. A chain or tape measure was used to perform the actual distance measurement between the theodolite and the point to be established. In addition to this, the surveyor used a telescopic device called a horizontal level and a graduated rod for determining the actual elevation of a point or location in question.
The art of surveying has advanced considerably. A parallel light or energy beam is now used to determine a precise distance measurement. This beam can have an infrared or laser energy source. In conjunction with this beam, a retro-reflective device, such as a corner cube prism is arranged whereby the energy beam is reflected as a parallel colinear beam of energy back to a receiver. Through phase angle measurements and timing circuits the exact distance between the transmitter and reflector can be precisely determined and displayed. This type of electronic instrument has greatly improved the accuracy that is possible by a surveyor in the taking of measurements and the setting of points. Typically, these electronic distance measuring devices are being used to provide range measurements with respect to the remotely located reflecting devices or prisms which may be as far away as two or three thousand feet from the device.
In recent years, the electronic measuring device has been combined with a typical transit/theodolite instrument and level or vertical collimator into a combination instrument now called a "Total Station=38 . Well known manufacturers of these instruments include such companies as Hewlett Packard, The Lietz Company and AGA Corporation. The total station normally includes the optical telescope of the theodolite which has a standard magnification of 30 power. In addition to the electronic distance measuring device, the total station includes the ability to visually measure vertical as well as horizontal angles. Thus, this single instrument is able to perform calculations and accomplish all of the functions that are required by the surveyor in performing his services.
Although the total station is capable of performing the necessary functions and provide the required accuracies in measuring both angles and distance, a major problem with this instrument is the retro-reflective device and visual target that are required for the sighting of the instrument as well as the distance measuring function. Up to now, the retro-reflective device used with this instrument has been a corner cube prism, which is mounted or supported on a slender prism pole and held or controlled by the surveyor's associate. Two levelling bubbles mounted on the pole and positioned in intersecting planes are used to aide the associate in holding the prism in a vertical position. One of the problems with this type of prism has been the inability of the surveyor to accurately sight the center of the prism when it is a considerable distance from the total station.
Even though most of the actual distance measurements and point sightings are within eight hundred to one thousand feet of the surveying instrument, it is still difficult to visually sight on such a small object. Because of this, a number of enlarged planar visual targets having various types of sighting indicia or patterns painted or embossed on the face of the target are attached to or positioned to surround the prism to aide the surveyor in sighting the retro-reflective device. In order to be able to properly use the prism it is necessary to position the prism and target perpendicular to the line of sight of the total station and to extrapolate the alignment center of the visual target which essentially causes the surveyor to guess at the exact center of the target which is usually occupied by the prism. This type of target and the fact that the instrument requires the use of the reflective prism creates a number of inaccuracies in the sighting function that is performed by the total station and in turn the work performed by the surveyor.
As a result, an object of the present invention is to provide a combination prism which also performs as a precise center target for both the distance measuring function and the angular sighting function.
Another object of the present invention is to provide an economical retro-reflective device for surveyors which eliminates the necessity for an additional large planar target for surrounding the prism which is quite fragile and which produces inaccuracies in the visual angular measurements.
Another object of the present invention is to provide an exterior sighting target which can be used with the retro-reflective device of this invention which will eliminate the inaccuracies which have been found with the existing exterior sighting targets.